Line-of-sight optical communication systems (LSOCs) are becoming increasingly important in a wide variety of applications. A typical LSOC comprises a laser transmitter for transmitting a modulated narrow beam of light through space or the Earth's atmosphere to a photodetector receiver. In space, line-of-sight optical communication to and from satellites is generally faster than radio communication and uses lighter equipment. On Earth, line-of-sight optical communication is free of spectrum regulation and is less susceptible to eavesdropping than radio. Moreover, the laser beam transmitters can be modulated at extremely high rates.
One difficulty that has delayed the full potential of LSOC systems is the necessity that the receiver and transmitter be very accurately pointed at one another to establish the desired link. Moreover, even after the link is established, the accuracy of pointing must be monitored and maintained over time. The system must adjust to the movement of satellites, the rotation of the Earth, and the vibration and sway of the transmitting and receiving stations. Any loss of pointing accuracy leads to a reduction or loss of signal that can substantially impair communication.
While there have been a variety of approaches to achieving the desired accuracy, none have proven completely satisfactory. One approach in terrestrial applications is to affix a spotting telescope to a data receiving telescope. The spotting scope allows a human operator to visually direct the receiver toward the transmitter. This approach, however, adds extra hardware to the system and requires human intervention. Moreover it requires precise alignment of the spotting scope with the axis of the data scope.
In space applications, added optical beacons can be used to direct added track sensors on the receiving satellite. The added beacons and sensors, however, increase the weight of the equipment and impose additional power demands on the satellite.
An alternative approach uses an imaging array, such as a CCD camera, to capture some of the light from the receiving telescope. The array feeds a signal to a computer which, in turn, analyzes the image and directs the telescope to the transmitter. This approach, however, requires that a beam splitter be placed in the main optical path, reducing the power available for data reception. It also introduces a second optical path that requires precise alignment of the beam splitter, the optical path to the detector and the optical path to the images.
Nor has the prior art found a completely satisfactory solution to the problem of maintaining pointing accuracy after lock-on. In the usual approach, a beam splitter is placed in the optical path and a fraction of the beam is diverted to a detector, such as a quadrant detector or imaging array, that is sensitive to the location of a laser spot on its surface. U.S. Pat. No. 5,770,850 granted to Bowen et al in 1998 and U.S. Pat. No. 6,522,440 granted to Poon et al in 2003 exemplify such systems. The beam splitter, however, consumes power and also introduces an additional optical path which must be precisely aligned. Furthermore, the position detector must be precisely located so that when the incoming data beam is perfectly centered on the data receiving photodetector, the split-off beam lies at the precise center of the position detector.
To avoid the latter difficulties in transmitter tracking, it has been suggested that the quadrant detector have a hole or window at its center, so that the majority of the incoming beam may pass through the hole or transmission window to a data detector located behind the detector. See U.S. Pat. No. 6,493,490 granted to Steiger in 2002 and U.S. Pat. No. 5,790,291 granted to Britz in 1998. This proposal, however, requires assembling the data photodetector with the quadrant detector and positioning the data detector accurately at the center of the hole or window. It also requires a separate solution to the problem of transmitter recognition and acquisition.
Accordingly there is a need for improved LSOC systems including a compact receiver that can be readily and continuously aligned with the transmitter.